Yes

No its not possible.
Because in function overloading the return type is not play 
any role.

function signature (name mangling) is done based on function
name and no. of parameter.we can't diff. by return type
because when function call there will be ambiguty.

No,writing no. of functions with the same name but 
different argument is called as function overloading.
The arguments may be differ in nos,type etc.
The return type may or may not be same.

Generally no but.......... ADA(Language) supports it
..............Source Wikipedia search for polymorph ism

Yes, it is possible here are the examples

create or replace function funbool(b varchar2)
return boolean
as
begin
If b is not null then
return true;
else
return false;
end if;
end;

create or replace function funbool(b varchar2)
return number
is
begin
if b is null then
Dbms_output.put_line( 'false');
else
Dbms_output.put_line( 'true');
end if;
Return b;
end;

There is a catch in the question. We can have function overloading with same number of arguments with different return types. One step ahead, we can have function overloading with same number of arguments provided the type and order of the arguments are not same irrespective of return type.